1. Field of the Invention
The invention is related to the field of valves, and more particularly, to proportional valves.
2. Description of the Prior Art
A proportional valve is a variable flow valve that may output a continuous range of flow rates. In the proportional valve, the valve opening amount may be proportional to a solenoid current in an associated valve actuation solenoid. As a result, the proportional valve can provide a regulated fluid flow ranging from a zero flow to a valve maximum flow.
FIG. 1 shows prior art proportional valve. In the prior art proportional valve, a poppet portion of a movable solenoid armature contacts and blocks a valve seat. The poppet may be moved away from the valve seat in order to open the valve and permit flow. The poppet may be moved away from the valve seat by any distance up to a maximum opening displacement.
The proportional valve may comprise a solenoid valve that generates magnetic force in order to open the valve armature. The magnetic force operates in opposition to a biasing force generated by a spring, with the biasing force holding the valve armature in a normally closed position in the proportional valve embodiment shown in the figure. The magnet force must overcome the spring force in order to move the valve armature/poppet in an opening direction.
However, when the fluid is flowing, such as when the valve armature is open even a small amount, then fluid static and/or dynamic pressures may also act on the valve armature. For small Mach numbers, Bernoulli's equation for compressible flow may be used to determine the static pressure of the fluid in the prior art valve. Bernoulli's equation comprises:ν2/2+ψ+p/ρ=constant
The static fluid pressure is defined as (P) and the fluid velocity is (ν). The (ρ) term is the density of the fluid. Here, it is assumed that (ρ) is constant. The (ψ) term is the force potential at a point considered on a streamline, e.g., is the force potential due to the Earth's gravity where (g) is an acceleration due to gravity and z is an elevation value above a reference plane, wherein w is defined as ψ=gz. The dynamic fluid pressure is defined as (ρ/2*ν2).
The Coanda effect comprises the tendency of a moving jet of fluid to be attracted to a nearby surface, or a surface to be pulled toward the jet of fluid. The Coanda effect influences the movement of the prior art valve armature/poppet. When the fluid is not flowing through the valve, then there is only static fluid pressure present in the valve. When the fluid is flowing, the static pressure decreases and the dynamic fluid pressure increases. The valve armature, which is at the region where the dynamic pressure occurs, will experience a static pressure force in the direction of the valve seat, and consequently will have a tendency to stick. This Coanda effect is only active or effective at comparatively small opening displacements of the valve armature.
As the armature moves in an opening direction and opens the orifice A2, the effective orifice area A2′ decreases, which means that the opening force F=((A1−A2′)*P1) grows larger and pushes the armature toward the fully open position. As a result, a stable force balance is only possible at small valve armature/poppet openings, when the flow force (i.e., the Coanda effect) helps to stabilize the valve armature.
Additionally, when the flow is high, there is a pressure drop from P2 to P2′. However, the pressure P2′ will increase when the fluid flow becomes higher. The straight and regular outflow bore of the prior art proportional valve therefore creates an additional opening force (A2′*P2′). This additional opening force is the root cause of the sticking effect on the valve armature when the valve armature is in the fully opened position.
The prior art proportional valve has drawbacks. The amount of actuation distance between the poppet and the valve seat can affect the flow in the prior art proportional valve. When the prior art poppet is moved upward by the energization of a solenoid coil, then an effective pressure area is increased and causes an increased opening force to act on the poppet surface area and create an upward force in the figure. The flow can change a dynamic pressure underneath the prior art poppet member, and the dynamic pressure can affect the poppet opening force and the poppet opening displacement.
The resulting force makes precise control of the prior art proportional poppet valve difficult, wherein the prior art proportional poppet valve may not maintain accurate proportional flow, as the increased dynamic pressure underneath the valve poppet may open the poppet by an additional, unwanted amount. The pressure can therefore affect the flow rate of fluid through the prior art proportional valve, making the prior art proportional valve less accurate and less reliable.
The prior art proportional valve may experience a condition where the poppet is not capable of being controlled past a certain opening distance, and instead the poppet may go from a partial actuation to a fully open actuation, where the solenoid opening force is aided by the increased dynamic pressure. As a result, the solenoid loses the ability to make further small changes in poppet displacement. In addition, the dynamic pressure force may operate to stick the poppet in the fully open position under some conditions.
What is needed, therefore, is an improved proportional valve wherein fluid flow does not significantly affect the position or displacement of the poppet.